Submitted to: Acta Horticulturae
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/1/2017
Publication Date: 2/1/2018
Citation: Altland, J.E., Locke, J.C., Boldt, J.K. 2018. Pyrolysis temperature and heating time affect rice hull biochar properties. Acta Horticulturae. 1191:145-152.
Interpretive Summary: Most biochar research has been conducted on very specific biochar materials, making generalizations about biochar properties difficult. Properties of biochar are most affected by the starting feedstock and the pyrolysis regime. The feedstock tends to be regional due to the expense of shipping or hauling biomass materials to the pyrolysis unit. Pyrolysis regime is also very specific and often implemented with the goal of extracting a specific range of thermally decomposed oils and gasses from the biomass, while the biochar component is largely considered a waste byproduct for disposal. The collective body of research on biochar applications to soilless substrates may show some consistent trends; however, a more systematic approach to how feedstock and pyrolysis regime affect the properties of biochar is warranted. The objective of this research was to determine the effect of pyrolysis temperature and exposure duration (time) on rice hull biochar chemical properties and their impact on nursery and greenhouse substrates. Biochar derived from rice hulls contained a measurable amount of macronutrients and micronutrients. With the exception of sulfur all nutrients (phosphorus (P), potassium (K), calcium, magnesium, iron, manganese, copper, molybdenum, and zinc) increased in concentration proportionally with the reduction in mass caused by thermal decomposition of the carbon, hydrogen, and oxygen. Of the total P and K in rice hull biochar, a high percentage (36% to 100%) was water soluble and readily available for plant uptake. Phosphorus and K concentrations in biochar heated to 600 °C are high enough to be detectable in leachates of a typical greenhouse substrate.
Technical Abstract: Rice hulls are an abundant byproduct of rice production, and thus potentially useful as a feedstock in pyrolysis systems that generate biofuels and biochar. Pyrolysis temperatures affect the relative mass of biofuel and biochar, as well as their properties. The objective of this research was to determine the effect of pyrolysis temperature and exposure duration (time) on rice hull biochar properties, and subsequently, their impact on nursery and greenhouse substrates. Parboiled rice hulls were subjected to temperatures of 200, 400, or 600 °C for 1, 2, or 4 hours. Mass of rice hulls were reduced 12%, 57%, and 63% at temperatures of 200, 400, and 600 °C, respectively. Elemental composition generally increased proportionally to the decrease in mass, as carbon, hydrogen, and oxygen were burned off and other macro- and micro-nutrients were concentrated in the resultant biochar (with the exception of S). Phosphate and potassium release generally increased with temperature and time, and was greatest from biochar heated to 600 °C. Calcium, magnesium, and micronutrient concentrations were generally low regardless of pyrolysis treatment.